I posted this question on ToneWay's Facebook Group page to elicit advice on how to deal with difficulties that may crop up in a jam. Here are a few of the responses. I'll add more. Please DON'T HESITATE to offer any suggestions based on your personal experience. Thank you!

QUESTION: In a public jam that you are holding, how do you deal with someone who is very difficult to have around? (Ex. angry, or has very off timing, or is insensitive to others and what's needed to make a jam work)? How do you talk to that person?

From Dianne Porter:

At our Jammalong we have guidelines on our web page that indicate prima-donna's won't fit in as our group is about all levels of musicians playing together. This is reinforced to newcomers and people are reminded of this if they get too pushy.

We encourage participants to come with a song to share and the rule is the person calling the song sets the pace and the key. If a song is too hard to share with others we say so after trying and the song fails. Song failure is part of the learning curve for everyone as some songs do not share well.

My husband and I gently police the group if needed and have found after a while the members of the group will take on doing this without promoting. We have had people who have a different agenda and they have tried to push it on others - thats when my husband's and my role as leaders comes forward and we do deal with people like that outside the group time as a general rule by calling the offenders attention to what we are offering and sharing with them how their approach may need to change to accommodate the other members of the group who are also learning.

We do find people self select out of the group when it no longer meets their needs. Our foundation as the group leaders in our case is the agreed reason my husband and I set the group up – we want to play music with other people not be played at. There is a difference. Some folk play music at us – they do not really fit in with what we are doing and we tell them that.

From Carolyn Loomes:

I had had this issue a couple times, being a beginner myself i had someone didnt want to lean jst complain and rude, I discontinued this at church as it was unpleasant for all and when you have one like this he usually sidles up to a buddy so both of them very difficult about the numbers 1, 4, 5, etc…now this number system is an advantage and if ppl cant use the nashville number system it means they cant “Hang” i have had someone else playing with me in nursing homes, speaking innappropriately… now i dont advertise when we play and have a core group pvt message and pop up like militia working to improve. Adapt and manoever around them, lose them some how only way.

From Mariska Jen Miessler:

I AM VERY protective of my space and the ENERGY around me. I have (politely) AND after several - private talks - walked friends out of my gate.

If you are hosting the event… Perhaps it's time for an honest conversation? You may lose that friendship however, you may gain far more friends at your jam sessions and, they'll be the kind of people that energize you rather than drain you. As well, they may be very grateful for your strength in protecting not only your space but the space of your guests

There is a big difference between a jam and an open mic event. We have people who come to a jam and play complicated, or self-written with a 5 verse story line, etc. We remind them that we have all levels of players that are looking to play with everyone else-hence the word jam. If you want to show off, go somewhere to an open mic!

I think it's very important to address the issue directly, as soon as possible, and every time it happens. This doesn't have to just come from the facilitators–anyone can ask a jam hog to wait his/her turn to start tunes and encourage newer or shy jammers to start or suggest a tune. It may be unpleasant for a moment, but the whole group can steer the attention away from the jam hog. If someone does not respond to the group vibe then facilitators need to step in and talk with the offender individually.

just went through this at my jam I have been hosting now going on 7 years.
I sent an E Mail to everyone on the list and told them the jam was straying from our roots and gave them the set of rules we came up with when we first started.
This month will be our first meeting since that and I am expecting some to not go or go and try to start a problem.
For my group there are many things to think about it is an open jam for one and secondly we are also playing for the elderly at a nursing home so if trouble starts I will have to be quick and to the point which is no problem for me.
And lastly if need be I will tell them they are no longer welcomed here and then explain to the head of the homes coordinator and she is a very understanding lady but I will try everything I can to make them see the light if you will first.

with that said sometimes in life ties need to be cut for the good of the group as a jam is not about any one person

I have noticed that the big difference between local jammers and a professional is that the professional knows when to shut up.

I have been in jams where someone with a lead instrument such as a banjo, fiddle or mandolin plays melody continually from start to finish. They have no idea that this is very irritating to the other members of the jam. Should someone say something? Or, do we put up with it because it is an open jam and no one is the boss?

It is sad indeed that one person is allowed to ruin a good time for the rest of the group.